refactor: celery task result endpoint crashes on task failure

When a Celery task fails, `result.ready()` evaluates to `True` but `result.successful()` is `False`. 
Calling `result.get()` without `propagate=False` on a failed task will re-raise the task's exception, causing the Flask endpoint to crash with a 500 Internal Server Error instead of returning the task's failed status. 
Additionally, if `result.result` is an Exception object, returning it directly in the dictionary will cause a `TypeError` during JSON serialization.


Affected files: views.py

Signed-off-by: Tang Vu <vuminhtang2212@gmail.com>

examples/celery/src/task_app/views.py

@@ -11,10 +11,12 @@ bp = Blueprint("tasks", __name__, url_prefix="/tasks")
 def result(id: str) -> dict[str, object]:
     result = AsyncResult(id)
     ready = result.ready()
+    successful = result.successful() if ready else None
+    value = result.get() if successful else result.result
     return {
         "ready": ready,
-        "successful": result.successful() if ready else None,
-        "value": result.get() if ready else result.result,
+        "successful": successful,
+        "value": str(value) if isinstance(value, Exception) else value,
     }